1. Field of the Invention
The present invention relates to novel piezoelectric resonators and electronic components containing such novel piezoelectric resonators, and more particularly, to a piezoelectric resonator which maximizes the efficient use of mechanical resonance of a novel piezoelectric member, and electronic components containing the novel piezoelectric resonator, such as a ladder filter, an oscillator, a discriminator, and a filter.
2. Description of the Related Art
FIG. 15 is a perspective view of a novel piezoelectric resonator which the applicant has developed but has not yet published. FIG. 16 is a view showing the internal structure of the piezoelectric resonator shown in FIG. 1. A piezoelectric resonator 1 includes a base member 2 having, for example, a substantially rectangular-parallelpiped shape. The base member 2 is preferably formed by integrally laminating a plurality of piezoelectric layers with electrodes 3 disposed therebetween and also disposed preferably on external surfaces of the piezoelectric layers. The piezoelectric layers are preferably made from piezoelectric ceramic. The electrodes 3 are disposed to be substantially perpendicular to a longitudinal direction of the base member 2 and they preferably have a certain space therebetween. The electrodes 3 are preferably not disposed near two opposite ends of the base member 2 in the longitudinal direction.
On opposite side surfaces of the base member 2, a plurality of insulating films 5 and 6 are provided, respectively. On the one side surface of the base member 2, alternate exposed portions of the electrodes 3 are covered by the insulating film 5. On the other side surface of the base member 2, alternate exposed portions of the electrodes 3 which are not covered by the insulating film 5 on the above-described side surface are covered by the insulating film 6.
On the side surfaces of the base member 2 on which the insulating films 5 and 6 are provided, that is, connection sections, external electrodes 7 and 8 are disposed. Therefore, the external electrode 7 is connected to electrodes 3 which are not covered by the insulating film 5 and the external electrode 8 is connected to electrodes 3 which are not covered by the insulating film 6. In other words, adjacent electrodes among the electrodes 3 are connected to the external electrodes 7 and 8, respectively. The external electrodes 7 and 8 are used as input and output electrodes.
In the piezoelectric resonator 1, the piezoelectric layers are preferably polarized in opposite directions at both sides of an electrode 3 in the longitudinal direction of the base member 2 as shown in FIG. 16. An active vibrating section 4 is defined as shown by hatching in FIG. 15.
Since the piezoelectric resonator shown in FIGS. 15 and 16 is a stiffened type resonator, it has a larger electromagnetic coupling coefficient than a conventional unstiffened piezoelectric resonator. It also has a relatively large .DELTA.F, which is a difference between the resonant frequency Fr and the antiresonant frequency Fa. A resonator having a large .DELTA.F is well suited for use in a wide-frequency-band filter.
A narrow-frequency-band filter and an oscillator may, however, require a small .DELTA.F. To obtain a small .DELTA.F, the area of each electrode, or the size or thickness of the piezoelectric layers must be changed in the piezoelectric resonator 1 shown in FIGS. 15 and 16, which causes difficulties in design and manufacturing.